1. Field of the Invention
This invention relates to an electrical bushing used with the lead-wire portion of tank shaped electrical apparatus, wherein the tank has accommodations for a super-high voltage transfer and switching device of the 500 kv and higher classification and wherein the tank is charged with an insulating material such as an insulating gas, oil or the like.
2. Description of the Prior Art
High voltage electrical apparatus may be used in an environment where damage due to airborne pollutants, such as salt, is high. These apparatus utilize bushings with long porcelain tubes, thus making the surface leakage distance long, for connection to associated overhead lines. The bushings must be capable of withstanding such harsh environments. In the case where electrical apparatus are employed in a district having a high frequency of occurrence of earthquakes, such as in Japan, they are always exposed to dangers due to earthquakes and therefore are designed with emphasis on seismic strength. When the electrical apparatus experiences an earthquake, the amplification of the earthquake experienced by the bushings is dependent upon the installation position, the foundation of the equipment, the tank portions, the seat for mounting the bushings, etc. Also, the number of proper vibration, or resonant frequency, is determined by the relationship between the weight distribution and the rigidity of the bushing. If the frequency of an earthquake approximates or corresponds to the resonant frequency of the apparatus, then a resonant phenomenon is developed such that vibrations are amplified on the foundation of the bushing, and on portions of the tank and the seat for mounting the bushing. This resonant phenomenon results in very high vibrations which are applied thereto until the breaking strength of the bushing is exceeded resulting in the breaking of the porcelain tube.
The frequency of earthquakes ranges generally from one to ten hertz. Bushings disposed on electrical apparatus of the 200 kv and higher classification may have a resonant frequency equal to or less than ten hertz which corresponds to the frequency of earthquakes. For its dimensions up to the order of five meters, the porcelain tubes for bushings of these classifications of apparatus have a sufficient seismic strength such that their breaking strength is not exceeded by the greatest earthquakes experienced in the past. However, for the 500 kv and higher classification the use of long porcelain tubes that are of the environmentally resistive type results in a resonant frequency not higher than a few hertz which corresponds to the frequency of earthquakes with a high probability. Thus it is possible for the porcelain tube to break upon the occurrence of great earthquakes. It is difficult to increase the seismic strength of the porcelain tubes. When the 1000 kv class is put to practical use, there is considered as a plan of increasing the seismic strength a method of reinforcing the bushing in three or four directions from its extremity by means of stay insulators or the like. In this case, the vibration of the stay insulators becomes a chordal vibration and a phenomenon is developed which includes an overlapped vibration different from that of the bushing portion. This makes an analysis of the seismic strength difficult and leaves questions about the reliability. Also, it is necessary to consider the flashover voltage with the parallel connection of stay insulators as portions of the bushing apparatus. The adhesion of soils is different between the bushing portion and the stay portion, the stay portion having a smaller diameter is generally apt to be soiled. In any case, it is required to determine the magnitude of the flashover voltage in the parallel state. With these aspects in view, the reliability is also reduced.